The invention relates to a cathode-ray tube and, more particularly, to a cathode-ray tube having an electron gun with a cathode of a high current density in which stable electron emission characteristics are obtained for a long time.
Various cathode-ray tubes such as television picture tubes and image display electron tubes used as monitor tubes of information processing terminals have an electron gun for emitting one or a plurality of electron beams at one end of a vacuum envelope and a phosphor screen obtained by coating a fluorescent material film of one or a plurality of colors onto an inner surface at the other end of the vacuum envelope so that an electron beam emitted from the electron gun is two-dimensionally scanned by a magnetic field generated by a deflecting yoke attached to the outside of the vacuum envelope, thereby displaying a desired image.
For such a kind of recent cathode-ray tube, high definition of a display image is required in association with realization of a variety of display information, a high density of display information, and the like. To realize high definition of a display image, it is necessary to remarkably improve focusing characteristics of the electron beam.
As means for improving the focusing characteristics to satisfy the requirement of the high definition mentioned above, it is considered to reduce an electron beam passage hole of a first grid electrode of the electron gun.
However, there arise a limitation on the reduction of the dimension of the diameter of the electron beam passage hole of the first grid electrode due to a limitation on a cut-off voltage, a limitation of an elecrode manufacturing precision, and a limitation of a cathode loading. Particularly, the cathode loading relates to the life reliability of the cathode-ray tube and exerts a large influence in case of deciding the hole diameter of the first grid electrode.
Therefore, the reduction of the hole diameter of the first grid electrode is limited to no less than 0.40 mm except for a combined use of an impregnated cathode suitable for a high current density operation. The impregnated cathode, however, has problems such that the number of manufacturing steps is large and it is expensive.
A cathode-ray tube having a cathode with an electron emissive material layer to solve such problems has been disclosed in U.S. Pat. No. 5,216,320 issued Jun. 1, 1993, and assigned to the same assignee as that of the present invention.
FIG. 1 is a cross sectional view for explaining a structure similar to that of a cathode shown in the above U.S. patent. Reference numeral 13 denotes a cylindrical cathode sleeve; 14 a cap-shaped metal base; 15 an electron emissive material layer; 16 a heater; 19 a first layer made of an oxide of alkaline earth metal; and 20 a second layer made of an oxide of alkaline earth metal containing a rare earth metal (for example, barium scandate Ba.sub.2 Sc.sub.2 O.sub.5).
The cap-shaped metal base 14, sealing one end of the cylindrical cathode sleeve 13, is made of a material made of refractory metal, for example, nickel (Ni) as a main component and containing therein a reduction metal of a small quantity of silicon (Si) or magnesium (Mg). The heater 16 is placed in the cathode sleeve 13, thereby constructing an indirectly heated cathode.
The electron emissive material layer 15 having a double layer structure is coated or formed on the upper surface of the metal base 14. The electron emissive material layer 15 includes the first layer 19 made of an oxide of alkaline earth metal coated or formed so as to be in contact with the upper surface of the metal base 14 and the second layer 20 made of an oxide of alkaline earth metal which is coated or formed on the surface of the first layer 19 and contains an oxide of rare earth metal such as barium scandate or the like.
The electron emissive material layer 15 with the above structure may be manufactured as follows. A first layer of a carbonate of alkaline earth metal is first formed on the upper surface of the metal base 14. A second layer of a carbonate of alkaline earth metal containing an oxide of rare earth metal such as barium scandate (Ba.sub.2 Sc.sub.2 O.sub.5) or the like is formed on the first layer. After that, the carbonate of alkaline earth metal of each of the first and second layers is changed to an oxide of alkaline earth metal by a thermal decomposition at the time of a heat treatment in a manufacturing step of the cathode-ray tube, thereby obtaining the electron emissive material layer 15 comprising the first layer 19 and the second layer 20 laminated on the first layer 19.
According to the structure of the electron emissive material layer 15 as mentioned above, the second layer 20 made of the oxide of alkaline earth metal formed on the electron emissive surface side of the electron emissive material layer 15 and containing the oxide of rare earth metal such as barium scandate or the like binds free barium (Ba) formed by a reduction element in the metal substance 14 to the second layer 20 and maintains free barium in the electron emissive material layer 15 so as to be in a high density state.
Thus, even when the cathode operates in a high current density state, an amount of Joule heat generated in the electron emissive material layer 15 is reduced and a degree of evaporation of barium also decreases.
As mentioned above, even when the cathode shown in FIG. 1 is operated in a high density current state, for example, in a large current state exceeding 2 A/cm.sup.2, the reduction of current emission is small and the cathode of a long life can be realized.